Coupled resonator filters can be made using bent or twisted electromechanical resonators, or electromechanical resonators with bulk waves. In this type of filter, the dimensions of the resonant elements need to decrease as the resonant frequency increases. This causes manufacturing, cost and reliability problems. Furthermore, the input/output impedances of these filters are very high (about several kohms) due to the nature of the detection made (conventionally a capacitance measurement).
BAW (Bulk Acoustic Wave) filters can be made from coupled bulk acoustic wave piezoelectric resonators. The signal to be filtered is propagated vertically in stacked resonant layers, directly or by an acoustic propagation medium, with layers on top of each other. They are called Stacked Crystal Filters (SCF) and Coupled Resonators Filters (CRF).
An example SCF described in U.S. Pat. No. 5,621,833 provides a fairly narrow passband at the output (for example, a passband of about 50 MHz centered around f=1.5 GHz), but too wide for channel filtering (for example, requires a passband of less than 5 MHz). Finally, when it is required to filter at intermediate frequencies, i.e., at frequencies between about 1 MHZ and 500 MHz, the thicknesses necessary for this type of filter become much too large for manufacturing.
An example CRF is described in U.S. Pat. No. 6,720,844. The passband obtained with this type of filter is wider than the passband obtained with an SCF (for example, a passband of about 70 MHz centered around f=1.5 GHz). Therefore, the width of the 4 passband obtained at the output is also too large for channel filtering. The dimensions necessary for this type of filter are also much too large when it is required to filter at intermediate frequencies.
Parasitic wavelengths for which the corresponding frequencies are present in the output spectra of some piezoelectric resonators filters originate from lateral resonances called Lamb waves. Recent work has made it possible to manufacture resonators using these Lamb waves. In particular, these resonators can give a high quality factor Q (about 2000) for a coupling coefficient K2 on the order of 0.8%. Such resonators are described in the publication by A. Volatier, G. Caruyer, and E. Defaÿ, “UHF-VHF Resonators Using Lamb Waves Co-integrated With Bulk Acoustic Wave Resonators”, IEEE Ultrasonics Symposium, September 2005.